Plant and method for producing ethylene

ABSTRACT

A plant for producing ethylene having: a reactor to effect oxidative coupling of methane, a work-up unit connected to the reactor to separate a first material stream produced by the reactor into a C 1−  material stream and an ethylene product stream and a separation unit connected to the work-up unit to separate the C 1−  material stream at least into a hydrogen-rich product stream and a hydrogen-lean residual gas stream. The plant also includes a steam cracker to produce ( 5 ) for producing an olefin-containing and hydrogen-containing crude gas stream. The steam cracker is connected to the work-up unit that separates the crude gas stream along with the first material stream into the C 1−  material stream and the ethylene product stream. A portion of the residual gas stream is recycled to the reactor. A process for producing ethylene using the plant is also described.

The invention relates to a plant for producing ethylene, in particular by oxidative coupling of methane according to the preamble of claim 1. The invention further relates to a process for producing ethylene.

Such a plant comprises a reactor into which oxygen and methane are introduced to effect oxidative coupling of methane. in this reaction methyl radicals are initially formed at high temperatures (about 600-880° C.) over a catalytic surface (e.g. two- or multicomponent metal oxide catalyst comprising alkali elements, alkaline earth elements and/or elements selected from the group of rare earths) in the reactor which then recombine in the gas phase to form ethane which is converted into ethylene in the further course of the reaction, a first material stream comprising at least ethane and ethylene being obtained as a result. The plant further comprises a work-up unit fluidly connected to the reactor and configured to separate the first material stream into at least a C¹⁻ material stream, comprising hydrocarbons having one carbon atom and further comprising CO and H₂, and into an ethylene product stream, said plant yet further comprising a separator (e.g. pressure swing adsorber) connected to the work-up unit and configured to separate the C¹⁻ material stream into a hydrogen-rich hydrogen product stream and a hydrogen-lean residual gas stream which is typically fired.

Another existing method of ethylene or olefin preparation is steam cracking. This comprises mixing a hydrocarbon-containing input with steam and typically passing the input gas thus formed through metallic tubes of a cracking furnace to effect cracking, said tubes being externally heated with burners to provide the necessary heat for the endothermic cracking process. The olefin- and hydrogen-containing crude gas stream thus obtained is typically purified and fractionated into the desired olefins, in particular ethylene.

The problem addressed by the present invention is that of further improving a plant and a process of the type cited at the beginning.

This problem is solved by a plant comprising the features of claim 1.

The invention accordingly provides a steam cracker for producing an olefin- and hydrogen-containing crude gas stream and connected to the work-up unit, wherein the work-up unit is configured to separate the crude gas stream along with the first material stream into at least the C¹⁻ material stream and the ethylene product stream, wherein the plant is configured to recycle at least a portion of the residual gas stream into the OCM reactor as input. The first material stream and the crude gas stream are preferably combined in the work-up unit and the individual streams may be prepurified prior to being combined. The C¹⁻ material stream and the ethylene product stream are then preferably removed from the combined stream.

The dovetailing in accordance with the invention of an OCM reactor with a steam cracker is advantageous since the hydrogen produced in the steam cracking and recycled into the OCM reactor provides energy in the OCM reactor through hydrogen combustion and also activates the catalyst in the reactor and reduces coking in the reactor.

The steam cracker which may comprise one or more cracking furnaces for steam cracking is preferably configured to crack hydrocarbon input, for example ethane, propane, natural gas condensates and/or relatively heavy inputs, for example naphtha, in the presence of steam to form the crude gas containing in particular hydrogen and ethylene.

The work-up unit is moreover preferably set up and provided to remove from the first material stream produced during oxidative coupling of methane and from the crude gas stream produced during steam cracking an ethane- and propane-rich material stream which is injected as input into the steam cracker which is similarly fluidly connected to the work-up unit.

It is furthermore preferable when the separation unit too is fluidly connected to the steam cracker, the plant being configured to inject at least a portion of the residual gas stream into the steam cracker which is configured to co-fire said portion of the residual gas stream to provide the heat required for the steam cracking of the aforementioned input (e.g. ethane and/or relatively heavy inputs such as naphtha) or to contribute to said heat.

The plant according to the invention may moreover be configured to recycle a substream of the hydrogen product stream into the work-up unit and to use said substream there in particular to hydrogenate hydrocarbons.

The problem addressed by the invention is moreover solved by a process for producing ethylene and comprising the features of claim 7.

The process according to the invention accordingly comprises introducing oxygen and methane into a reactor, effecting oxidative coupling of methane in the reactor to form a first material stream, steam cracking a hydrocarbon-containing input in a steam cracker to produce an olefin- and hydrogen-containing crude gas stream, separating said first material stream along with the crude gas stream into an ethylene product stream and a C¹⁻ material stream. in a work-up unit, separating the C¹⁻ material stream into a hydrogen product stream and a hydrogen-lean residual gas stream in a separation unit and recycling at least a substream of the residual gas stream into the reactor as input.

It is preferable to additionally remove from said first material stream and/or the crude gas stream an ethane- and propane-rich material stream which is passed into the steam cracking as input.

It is preferable when in addition at least a substream of the residual gas stream is co-fired to produce heat for said steam cracking.

It is moreover preferable when a substream of the hydrogen product stream is recycled into the work-up unit in order to be used there for hydrogenating hydrocarbons for example.

An illustrative embodiment of the invention will now be more particularly described with reference to the figure and the related description to elucidate further details and advantages of the invention.

FIG. 1 is a schematic diagram of a plant and a process according to the invention for producing ethylene.

FIG. 1 shows a plant 1 for preparing ethylene and comprising a reactor 2 for effecting oxidative coupling of methane (known as an OCM reactor), a work-up unit 3, a separation unit (e.g. pressure swing adorber) 4 and a steam cracker S comprising at least one cracking furnace.

CH₄ and O₂ are injected into reactor 2 to effect oxidative coupling of methane at elevated temperatures over a catalyst surface in reactor 2. This forms a first material stream S comprising, inter alia, ethylene and ethane.

Plant 1 further comprises a steam cracker 5 into which an input consisting of, for example, ethane and/or naphtha and steam is injected, said input being cracked in the steam cracker 5 to produce a crude gas stream C comprising, inter alia, olefins, in particular ethylene, and hydrogen.

In a work-up apparatus 3 provided downstream of reactor 2 said first optionally prepurified material stream S and the optionally prepurified crude gas stream C are combined and separated into a C₃₊ material stream (hydrocarbons comprising three or more carbon atoms), an ethylene product stream P, an ethane- and propane-rich material stream 5″ and a C¹⁻ material stream, the C¹⁻ material stream being injected into separation unit 4 to produce a hydrogen product stream H.

The ethane- and propane-rich material stream S″ removed in work-up apparatus 3 is passed into steam cracker 5 as additional input.

The C¹⁻ material stream is separated into a hydrogen product stream H and a hydrogen-lean residual gas stream 5′ in separation unit 3. This may be effected using, for example, pressure swing adsorption in which the C material stream is passed through at least one adsorber at elevated pressure, so that hydrogen passes through the at least one adsorber to form said hydrogen product stream H and the relatively heavy components, for example CH₄ and CO, are adsorbed by the at least one adsorber and desorbed at a relatively low pressure to afford the hydrogen-lean residual gas stream S′ this comprises in particular also hydrogen from a substream of the hydrogen product stream H which was used to purge the at least one adsorber).

The residual gas stream S′ is then returned to reactor 2 as input. A portion of the residual gas stream S′ may moreover be passed into the steam cracker 5 for co-ft ring to generate the heat required for the steam cracking or to contribute to said heat. Preference is moreover given to co-firing using CH₄ which may he diverted upstream of reactor 2.

Finally, a portion of the hydrogen product stream H may be used to hydrogenate components of the first material stream S and/or of the crude gas stream C (e.g. in work-up unit 3).

List of Reference Numerals

-   1 Plant -   2 OCM reactor -   3 Work-up unit -   4 Separation unit (e.g. pressure swing adsorber) -   5 Steam cracker -   I Input for steam cracking -   S First material stream -   S′ Hydrogen-containing residual gas stream -   S″ Ethane- and propane-rich material stream -   C Olefin- and hydrogen-containing crude gas stream -   P Ethylene product stream -   H Hydrogen product stream 

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 12. A plant for producing ethylene comprising: a reactor to effect oxidative coupling of methane and produce a first material stream; a steam cracker to produce an olefin-containing and hydrogen-containing crude gas stream; a work-up unit connected to the reactor and to the steam cracker to receive the first material stream and the crude gas stream and to produce a C¹⁻ material stream and an ethylene product stream; a separation unit connected to the work-up unit to separate the C¹⁻ material stream into a hydrogen-rich product stream and a hydrogen-lean residual gas stream; and a conduit connecting the separation unit to the reactor for recycling at least a portion of the residual gas stream from the separation unit to the reactor.
 13. The plant according to claim 12, wherein the steam cracker uses steam to crack an input of hydrocarbon, ethane, propane, natural gas condensates or naphtha to form the crude gas stream.
 14. The plant according to claim 12, wherein the work-up unit also produces an ethane-rich material stream and a propane-rich material.
 15. The plant according to claim 14, wherein the work-up unit is connected to the steam cracker to avow the ethane-rich material stream and propane-rich material stream to be introduced to the steam cracker as input.
 16. The plant according to claim 12, wherein the separation unit connected to the steam cracker to allow at least a portion of the residual gas stream to be introduced to the steam cracker and be fired as heat for the steam cracker.
 17. The plant according to claim 12 wherein the separation unit is a pressure swing adsorption unit.
 18. A method of producing ethylene comprising: introducing oxygen and methane into a reactor; effecting oxidative coupling of the methane in the reactor to form a first material stream; introducing a hydrocarbon-containing input to a steam cracker; steam cracking the hydrocarbon-containing input in the steam cracker to produce an olefin-containing and hydrogen-containing crude gas stream; introducing the first material stream and the crude gas stream into work-up unit; separating the first material stream and the crude gas stream using the work-up unit into an ethylene product stream and a C¹⁻ material stream; introducing the C¹⁻ material stream into a separation unit; separating the C¹⁻ material stream using the separation unit into a hydrogen product stream and a hydrogen-lean residual gas stream; and recycling at least a substream of the residual gas stream into the reactor.
 19. The method according to claim 18, further comprising: removing an ethane-rich and propane-rich material stream from said first material stream and the crude gas stream in the work-up unit; and recycling the ethane-rich and propane rich material stream to the steam cracker.
 20. The method according to claim 18, further comprising: recycling at least a substream of the residual gas stream to the steam cracker; and firing the recycled residual gas stream in the steam cracker to produce heat for steam cracking.
 21. The method according to claim 18, wherein separating the C¹⁻ material stream into a hydrogen product stream and a hydrogen-lean residual gas stream comprises separation by pressure swing adsorption in the separation unit.
 22. The method according to claim 18 wherein the input for the steam cracking is ethane, propane, natural gas condensates or naphtha. 